1. Field of the Invention
The present invention relates to light emitting apparatuses and more particularly to light emitting apparatuses formed from nitride semiconductors. A light emitting apparatus of the present invention refers to a semiconductor device or semiconductor chip mainly constituted by a nitride semiconductor substrate and semiconductor layers laminated thereon in some cases and also refers to a device including a semiconductor chip mounted on a mounting component and sealed with resin in some cases. Also, it may refer to the both in some cases. Further, a semiconductor chip is called a chip in some cases. Further, a substrate and epitaxial layers formed thereon in a chip are simply called a substrate in some cases.
2. Description of the Background Art
Currently, white light emitting diodes (LEDs) have been widely employed for illumination of compact electronic equipment such as portable information terminals and have the potentiality to be utilized for illumination for larger spaces or larger areas in the future. In order to utilize LEDs for illumination for larger spaces or larger areas, it is required to increase the light outputs of LEDs. Therefore, there is a need to enable flowing large currents through the electrodes of LEDs and overcome a problem of temperature increases caused by heat generation.
FIG. 59 illustrates the construction of a GaN-based LED which has been currently proposed (Japanese Patent Laying-Open No. 2003-8083). In this GaN-based LED, an n-type GaN layer 102 is provided on a sapphire substrate 101 and a quantum-well construction 103 is formed between n-type GaN layer 102 and a p-type GaN layer 104. Light emission occurs at quantum-well construction 103. On p-type GaN layer 104, a p-electrode 105 is formed to be in ohmic contact and on n-type GaN layer 102 an n-electrode 106 is formed to be in ohmic contact.
These p-electrode 105 and n-electrode 106 are connected to a mounting component 109 through solder balls 107, 108. The mounting component (sub-mount component) is formed from an Si substrate and provided with circuits for protection against surge voltages from the outside. Namely, in order to prevent large forward voltages or reverse voltages from being applied to the light emitting apparatus, the electrical power branching circuit for protecting the light emitting apparatus is constituted by Zener diodes, etc., placing emphasis on that main factors of circuit failure for semiconductor of a nitride of Ga, Al, In or other group III element are surge voltages such as transient voltage and static discharge. Protection against surge voltages will be described in detail later.
The above GaN-based LED is characterized in that (a1) p-type GaN layer 104 is down-mounted and (a2) n-electrode layer 106 is formed on n-type GaN layer 102 so that light is emitted from the backside of sapphire substrate 101. The construction of the GaN-based LED is significantly complicated as can be seen in FIG. 59. The reason that (a2) the n-electrode layer is formed on n-type GaN layer 102, which is the cause of such a complicated construction, is that sapphire substrate 101 is an insulator and the n-electrode can not be provided on the sapphire substrate.
As well as the above light emitting apparatus employing a sapphire substrate, there has been often suggested that circuits for protection against transient voltages and static discharge are provided in light emitting apparatuses employing GaAs-based, GaP-based, or GaN-based compound semiconductors for use in light emitting apparatus (Japanese Patent Laying-Open Nos. 2000-286457, 11-54801 and 11-220176). Particularly, in the case of GaN-based compound semiconductors, the breakdown strengths in the reverse direction are about 50 V and thus are low, and also the breakdown strengths for the forward voltage are only about 150 V. Therefore, there has been importance attached to providing an electrical power branching circuit for the above protection. Namely, the above GaN-based chip, etc., is formed on the Si substrate of the sub-mount and on the Si substrate the protection circuit including Zener diodes is constituted. It can be said that suggestions of many protection circuits as above are the proof of that main factors of circuit failure for semiconductor of a nitride of Ga, Al, In or other group III element are surge voltages such as transient voltage and static discharge.
Besides the light emitting apparatus provided with the above protection circuits, there have been known examples of forming a GaN-based light emitting apparatus on a conductive SiC substrate. Namely, LEDs constructed to emit light from a p-type GaN layer using a laminate construction of (n-electrode on the SiC substrate backside/SiC substrate/n-type GaN layer/quantum-well laminate construction (light emitting layer)/p-type GaN layer/p-electrode) have been also widely utilized.
In the case of GaN-based LEDs using a sapphire substrate illustrated in FIG. 59, the construction is complicated and the fabrication cast will be unavoidably increased. In order to develop the demand in large space illumination applications, LEDs are required to be cheap, and thus the aforementioned construction is not desirable. Furthermore, since p-electrode 105 and n-electrode 106 are placed on the down-mounted surface side, the areas of the electrodes, particularly the area of the p-electrode, is restricted. In order to enable flowing large currents to achieve high outputs, it is desired that the p-electrode has particularly a larger area. However, with the construction illustrated in FIG. 59, the area of the p-electrode is restricted and accordingly the light output is restricted. Further, in discharging heat generated in association with currents, providing two electrodes on one side is not desirable.
Furthermore, there is large resistance to currents flowing in the direction parallel to the substrate through n-type GaN layer 102, which causes heat generation and an increase of the driving voltage and, therefore, an increase of the power consumption. Particularly, if the thickness of the n-type GaN layer is reduced in order to shorten the film forming processes, the yield of exposure of the n-type GaN film will be significantly degraded, besides the above problem of heat generation and power consumption increases.
Further, as can be said in general for light emitting devices including the above light emitting device employing a sapphire substrate, since the heat radiating area is restricted and also the heat resistance (temperature increase caused by unit introduced energy per unit area) is large, the injection current per light emitting apparatus can not be increased. Particularly, in the case of using a sapphire substrate, since the area of the p-electrode is restricted, it is common to heat-design the apparatus with little margin.
Further, in the case of GaN-based LEDs using a sapphire substrate, since the heat radiating area is restricted, in order to reduce the electrical resistance as much as possible to reduce heat generation, it becomes necessary to employ a construction in which the p-electrode and the n-electrode are complicated in a comb shape to widen the contact area. Processing of such a comb-shaped electrode is not easy and the fabrication cost will be certainly increased.
As previously described, the design of heat conditions is basically important for light emitting apparatuses and when an attempt is made to obtain high outputs, there will be restriction due to heat conditions as above. Therefore, in order to alleviate this as much as possible, a complicated electrode shape must be employed.
Further, there are the following problems. In the case where a GaN-based light emitting apparatus formed on a sapphire substrate is down-mounted so that the back side of the sapphire substrate forms the light emitting surface, light with an incident angle greater than a predetermined angle experiences total internal reflection at the interface between the GaN layers which generate and propagate light and the sapphire substrate, and thus the light will not be emitted to the outside, since the refractive index of sapphire is about 1.8 and the refractive index of GaN is about 2.4. Namely, lights with incident angles θ≧sin−1 (1.8/2.4)≈4.2° will remain within the GaN layers and will not be emitted to the outside. Consequently, the light emission efficiency at the main surface of the sapphire substrate will be degraded. Although the problem of the light emission efficiency is important, there are still other problems. The light experienced the above total internal reflection propagates through the GaN layers and is emitted from the side portions of the GaN layers. Since the ratio of the light which experiences the above total internal reflection is significantly large and also the GaN layers are thin, the energy density of light emitted from the side portions becomes large. The sealing resin placed at the side portions of the GaN layers and irradiated with the light is damaged and this causes a problem of shortening the life of the light emitting apparatus.
In the case of a GaN-based LED having a construction of (SiC substrate backside n-electrode/SiC substrate/n-type GaN layer/quantum-well laminate construction (light emitting layer)/p-type GaN layer/p-electrode) for extracting light from the p-layers side, the light absorption ratio at the p-electrode is large and thus high light outputs can not be emitted to the outside efficiently. When an attempt is made to reduce the coverage ratio of the p-electrode and namely increase the opening ratio to increase the amount of light emission, currents can not be flowed through the entire p-type GaN layer because of the high electrical resistance of the p-type GaN layer. Consequently, light emission can not be activated through the entire quantum-well construction, and thus the light emission output is decreased. Further, the electrical resistance will be increased and this will induce a problem of heat generation and power supply capacity. Also, if the thickness of the p-type GaN layer is increased in order to flow currents uniformly through the entire p-type GaN layer, the light absorption at the p-type GaN layer becomes large and the output will be restricted.